The present invention relates to dental equipment, and more particularly, to measuring equipment for dental endodontics.
In root canal therapy, a dental probe, such as a reamer or file, is inserted into the canal and manipulated to remove undesired material A flexible filler substance is then placed in the root canal that is sealed with a rigid material. If the canal is not completely cleaned before filling and sealing, debris left inside the canal can prevent proper healing. The probe must therefore be inserted all the way to the apex of the root canal during cleaning to remove all debris. On the other hand, if the probe is inserted too deeply, the tool penetrates the jaw tissue, causing swelling and unnecessary trauma for the patient. It is therefore essential to precisely determine when the probe tip is located at the root canal apex so that the canal can be cleaned fully without excessive trauma to the patient.
Locating the apex is difficult because the narrow canal does not provide a clear viewing path and fluids can partially fill the canal. In one method, the probe is inserted into the canal and the tooth is x-rayed. In the x-ray image, the metal probe contrasts with the surrounding tooth and body tissue so that the positions of the probe tip and the apex can be compared. If the probe tip is not at the apex, it is inserted deeper into the canal and a new x-ray image is obtained. This method is unreliable, time-consuming, costly, and entails excessive exposure of the patient to X-ray radiation.
It is also known to locate a root apex by inserting a conductive probe into the root canal and placing an electrode in contact with the patient""s body, usually in or near the mouth. As the probe is moved through the canal towards the apex, electrical measurements across the probe and electrode are made. For example, U.S. Pat. No. 5,063,937 discloses an apex detector utilizing an AC signal at a single frequency. The probe tip is determined to be at the apex when the result of a calculation based upon these measurements is within a predetermined range. However, methods that utilize a single frequency AC signal for apex positioning are not accurate in the vicinity of the apex.
U.S. Pat. No. 5,096,419 to Kobayashi et al., discloses locating the apex by making impedance measurements at two frequencies as the probe is moved towards the apex. A detector determines that the probe tip as at the apex when the ratio of the two impedance measurements is within a predetermined range. U.S. Pat. No. 5,759,159 to Masreliez discloses locating a root apex based upon the amplitude or phase of voltage measurements across the probe and the electrode as the probe is moved towards the apex. However, amplitude and phase measurements are affected by inevitable signal distortion Moreover, the multiple frequency methods are accurate only near the apex and cannot indicate the probe position before the probe tip approaches the apex region.
With prior art apex locators, whether single or multiple frequency, the frequency or frequencies of the test signals, as well as the internal impedance, of the devices are fixed by the manufacturer, and cannot be changed during operation. However, the electrical parameters of the tissues of the root canal and jaw may vary significantly from patient to patient, from one tooth to another of the same patient, and even from one canal to another in the same tooth. Fixed test frequencies and internal impedance of the device obviously cannot be optimal for all cases. Hence, the precision afforded by these devices is limited.
In the following description and set of claims, two explicitly described, calculable or measurable variables are considered equivalent to each other when the two variables are substantially proportional to each other.
The present invention provides a method and device for locating the apex of a root canal. The device comprises two electrodes. A conductive probe inserted into a root canal forms the first electrode. A second electrode contacts the patient""s body, typically in the mouth area. An AC voltage test signal Vg is applied to a driver having an internal output impedance Zo. The internal output impedance of the device and the impedance of the tissues between the two electrodes form a parametric voltage divider. The test signal Vg may be a single frequency or a multi-frequency signal. The internal output impedance Zo may be kept constant during the measurements, or be made to vary. As the probe tip is moved through the canal towards the apex, voltage measurements Vl across the probe and electrode are continuously made and a microprocessor calculates one or more test scores based upon the voltage measurements. The current position of the probe in the canal is then determined on the basis of the test scores.
In the following description and set of claims, the portion of the root canal near the apex is referred to as xe2x80x9cregion Bxe2x80x9d of the canal. The rest of the canal is referred to herein as xe2x80x9cregion Axe2x80x9d of the canal. Determining the position of the probe tip may simply involve determining whether the tip is in region A, in region B, at the apex, or in the jaw tissue In this case, as the probe tip is moved towards the apex, the test score is compared to a first numerical value. When the test score is less than the first numerical value, the user is informed that the tip is in region A. When the test score first equals the first numerical value, the user is informed that the tip is in region B. When the test score first reaches a second numerical value, the user is informed that the probe tip is at the apex. When the test score exceeds the second numerical value, the user is informed that the tip has passed the apex and entered the jaw tissue. The position of the probe tip in the canal may be indicated to the user on a graphical or numerical display, or by means of a sensible signal such as a bell.
Alternatively, determining the position of the probe tip may involve continuously determining the distance of the probe tip from the apex. In this case, the location of the probe in the canal may be displayed continuously on a graphical or numerical display starting from the first insertion of the probe into the canal until the probe tip has reached the apex.
The test score may involve, for example, the root mean square (RMS) of the voltage measurements Vi. Unlike phase and amplitude measurements, RMS measurements are essentially unaffected by signal distortion. In one embodiment, a test signal Vg having a single frequency f1 is used while the internal output impedance Zo is kept constant, for example 3 Kohm. Vg(f1) may be, for example, 30-50 mV. As the probe is moved in the canal, the novel test score L1=S1(RMS(Vi(f1,P0))xe2x88x92RMS(Vi(f1,Pk))) is calculated where S1 is a scaling factor, and P0 and Pk designate, respectively, the initial and current probe positions inside the root canal. S1 may be, for example, about 75. This embodiment may make use of the unexpected finding that when the tip is in region A, the test score L1 is essentially a linear function of the probe tip position in the canal.
In another embodiment, a test signal Vg having two frequencies f1 and f2 is used while the internal output impedance Zo is kept constant. f1 and f2 may be, for example, in the 400-500 Hz and 8-10 kHz ranges, respectively As the probe is moved in the canal, the novel test score D1=S2(RMS(Vi(f1,Pk))xe2x88x92RMS(Vi(f2,Pk))) is calculated where S2 is another scaling factor. This embodiment may make use of the unexpected finding that in region B, D1 is a linear function of the distance between the probe tip and the apex.
In a most preferred embodiment, the test scores L1 and D1 are used simultaneously. The test score L1 is used to locate the probe tip when it is in region A. When L1 first satisfies a predetermined condition that may depend upon the simultaneously obtained test score D1, the probe tip has passed from region A to region B. The condition may be, for example that L1=D1(Pk)xe2x88x92D1(P0). The test score D1 is then used to determine the probe tip position. When the value of the test score D1 exceeds a first predetermined threshold T1, the probe tip is close to the apex, for instance, 1 mm from the apex. When the value of the score reaches a second predetermined threshold T2, the probe tip is at the apex. When value of the test score exceeds the second predetermined threshold T2, the probe tip has passed the apex. T1 and T2 may be, for example, 0.196 V, and 0.294V, respectively.
In another embodiment, Vi measurements are initially obtained using a testing signal Vg having two frequencies f1 and f2 and one or more test scores calculated. Vi measurements are then performed using a testing signal Vg having new frequencies f3 and f4 better suited to the specific case that are determined based upon the test scores obtained with the initial pair of frequencies. For example, initial frequencies f1 and f2 of 1 kHz and 8 kHz respectively may be selected. If a test score obtained using the frequencies f1 and f2 is less than a predetermined value, the frequencies are decreased. If the test scores exceeds this value, the frequencies are increased. The process may be repeated dynamically to arrive at an optimal pair of frequencies for the particular canal.
In yet another embodiment, voltage measurements are made at a fixed frequency but at two different output impedances Z1 and Z2 of the device. One or more test scores are calculated that may involve, for example, the amplitude, phase, or RMS of the voltage measurements. For example, the test scores L2=S1(RMS(V1(Z1,P0))xe2x88x92RMS(V1(Z1,Pk))) and D2=S2(RMS(Vi(Z1,Pk))xe2x88x92RMS(Vi(Z2,Pk))) may be used. The test score L2 is used to locate the probe tip when it is n region A. When L2 first satisfies a predetermined condition that may depend upon the simultaneously obtained test score D2, the probe tip has passed from region A to region B. The test score D2 is then used to determine the probe tip position. When the value of the test score D2 exceeds a first predetermined threshold T1, the probe tip is close to the apex, for instance, 1 mm from the apex. When the value of D2 reaches a second predetermined threshold T2, the probe tip is at the apex. When value of the test score exceeds the second predetermined threshold T2, the probe tip has passed the apex. T1 and T2 may be, for example, 0.196 V, and 0.294V, respectively.
In either embodiment the position of the probe tip in the canal may be displayed graphically and/or numerically to a user as the probe is moved in the canal. For example, the display may include an illustration of a tooth canal and a marker may; be introduced into the illustration indicating the position of the probe tip in the canal. Once the score D1 or D2 respectively exceeds a first threshold T1 indicating that the probe tip is close to the apex, the scale of the graphic display may be enlarged to provide a more accurate indication of position of the probe tip near the apex. A sensible signal such as a light or a bell may also be used to indicate that the probe is near or at the apex. A visual and/or audio signal may indicate that the probe tip has passed the apex. The invention thus provides a device for locating an apex of a root canal of a tooth, the root canal having a region A and a region B, comprising:
a a first electrode, the first electrode including a conductive probe adapted for insertion into the root canal, the probe having a tip;
b a second electrode configured to electrically contact a patient""s body;
c an AC voltage generator configured to provide an AC test voltage signal Vg having one or more frequencies;
d an AC voltage driver being an interface between the voltage generator and the electrodes;
e a voltage detector coupled to the first and second electrodes, the voltage detector being operative to detect an AC voltage Vi across the first and second electrodes;
f an electronic controller coupled to the voltage detector, the controller carrying out the steps of:
fa calculating one or more test scores, the one or more test scores each having a value that is a function of Vi;
fb determining, on the basis of the one or more test score values, the position of the probe tip in the canal; and
fc informing a user of the position of the probe tip in the canal.